This invention pertains to a silicon nitride (Si.sub.3 N.sub.4) ceramic body and a process for preparing the ceramic body.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, silicon nitride ceramics are resistant to chemical attack, particularly to oxidation. Because of these attributes, silicon nitride is useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Failure of silicon nitride ceramics is generally associated with brittleness and flaws. The object therefore is to prepare a silicon nitride ceramic with high fracture toughness (K.sub.IC) and strength. Fracture strength is directly proportional to the fracture toughness and inversely proportional to the square root of the flaw size. High fracture toughness combined with small flaw size is therefore highly desirable. Monolithic silicon nitride, however, has a relatively low fracture toughness of about 5 MPa (m).sup.1/2.